A sterile surgical instrument intended to be placed intraoperatively in a surgically-prepared mammary pocket to provide a volume measurement for the appropriate selection of a breast implant (mammary prosthesis). The device is typically composed of a silicone rubber shell with a single lumen, filled with a fixed amount of silicone gel, which is placed in the pocket and then removed. The sizer may vary in shape, profile, and volume. This is a reusable device intended to be resterilized between patient applications.